Making esters from alcohols and acids

In this experiment students investigate the reactions between a range of alcohols and acids on a test-tube scale, to produce small quantities of a variety of esters quickly.

Lesson organisation

As a class experiment this can be organised, if desired, as a class-cooperative investigation of the ability of a range of alcohols to react with a range of organic acids. Working groups could compare their results with others to build a general overview of this route to the formation of esters, with an interesting variety of smells.

Depending on the actual way the lesson is organised, this may be designed to take from 15 minutes to an hour.

Apparatus

Chemicals

Each working group will require:

Eye protection

Glass specimen tubes, 4 (Note 1)

Plastic dropping pipettes, access to adequate supply

Beaker (100 cm3 or 250 cm3), (Note 1)

Test-tubes, 4

Test-tube rack

Bunsen burner

Heat resistant mat

Tripod and gauze

Crucible tongs

Glacial (concentrated) ethanoic acid (CORROSIVE), about 2 cm3

Propanoic acid (CORROSIVE), about 2 cm3

Benzoic acid (HARMFUL), about 0.2 g

Concentrated sulfuric acid (CORROSIVE), 5-10 drops (Note 2)

Access to the following alcohols – about 10 drops of each required (Note 3):

1 The essential requirements for these tubes are: neutral borosilicate glass a wide flat base, so that they are stable when stood in a beaker. If not available, small test-tubes could be used instead, standing in a larger (250 cm3) beaker.

2 For younger students, prepare the specimen tubes by adding one drop of concentrated sulfuric acid to each. This minimises the risks involved with such students handling this substance. Advanced students may be reliable enough to prepare their own tubes in this way.

3 The alcohols (and organic acids) may be best provided as a central resource, away from flames, with a supply of plastic pipettes for each (or in small dropper bottles).

Procedure

a Add 10 drops of ethanoic acid (or propanoic acid) to the sulfuric acid in the specimen tube.

b Add 10 drops of ethanol (or other alcohol) to the mixture.

c Put about 10 cm3 of water into the 100 cm3 beaker. Carefully lower the tube into the beaker so that it stands upright.

d Heat the beaker gently on a tripod and gauze until the water begins to boil, then stop heating.

e Stand for 1 minute in the hot water. If the mixture in the tube boils, use the tongs to lift it out of the water until boiling stops, then return it to the hot water.

f After 1 minute, using tongs, carefully remove the tube and allow it to cool on the heat resistant mat.

g When cool, pour the mixture into a test-tube half-full of 0.5 M sodium carbonate solution. There will be some effervescence. Mix well by pouring back into the specimen tube – repeat if necessary. A layer of ester will separate and float on top of the aqueous layer.

h Smell the product by gently wafting the odour towards your nose with your hand – do not put your nose near the top of the tube!

i Repeat this procedure for up to three more different esters.

Compare the odours of the different esters prepared by your group and by other groups. Write word equations for each reaction, and (for advanced students) chemical equations using structural formulae.

For solid acids, the procedures in steps 1 and 2 need to be changed:j Add 1 cm3 of methanol (or other alcohol) to the sulfuric acid in the specimen tube.

k Weigh out 0.2 g of benzoic acid (or another solid acid, such as salicylic (2-hydroxybenzoic) acid (HARMFUL - see CLEAPSS Hazcard) and add it to the tube. Then proceed as above. Yields from solid acids are not as great, but odours are detectable and distinctive.

Teaching notes

This method is an updated version of the traditional test-tube scale approach to ester preparation, which minimises the risks involved in handling the reagents involved. For further information about this method of ester preparation, consult CLEAPSS Guidance Leaflet PS67-07 ‘Making esters’.

This method is only suitable for preparing small samples for characterisation by odour. Advanced students could scale up the quantities using larger test-tubes, but this would still not give sufficient product for isolation, characterisation by boiling point, or calculation of percentage yield.

Do NOT be tempted to use butanoic (butryric) acid, because of its very unpleasant odour (of rancid butter).

Health & Safety checked, 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

ADDITIONAL INFORMATION

This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.